1. Technical Field
This invention relates to amplifiers, and more specifically to power amplifiers used in wireless communication devices.
2. Related Art
It is desirable for the base station receivers in Code Division Multiple Access (CDMA) systems to receive roughly equal power from all CDMA handsets to help eliminate xe2x80x9cnear-far problemsxe2x80x9d associated with signal transmission and reception. Near-far problems arise from the fact that signals transmitted from near CDMA handsets (e.g handsets that are close to the base station) effectively xe2x80x9cdrown outxe2x80x9d the signal transmitted from the far CDMA handsets (e.g. handsets that are far from the base station). In order to eliminate these near-far problems, CDMA systems use power control where the base station rapidly samples the radio signal strength transmitted from each CDMA handset transmitter (e.g., each mobile unit transmitter) and then sends a power change command to the CDMA handset. The CDMA handset then adjusts the output power of its transmitter in response to the power change command by adjusting a gain, which is an incremental change in the output power associated with an incremental change in the power change command, associated with the output power of the CDMA handset. This allows the received signal at the base station from each CDMA handset transmitter to be approximately the same.
Some CDMA handsets operate in two distinct modes: open loop mode and closed loop mode. Prior to initiating a call, the CDMA handset is operating in the open loop mode. In open loop mode, the CDMA handset attempts to estimate an initial output power of the transmitter by adjusting a gain of a variable-gain amplifier in the CDMA handset. The gain is adjusted using a finite number of discrete, rather than continuous, iterations. Since, the gain resolution varies between different industry standards (e.g., the gain resolution for industry standard IS95A is different from the gain resolution for industry standard IS95B), and the desired output power is specified, this results in minimal variation of output power. Furthermore, using the same resolution for all the standards sometimes causes the minimal variation to exceed the tolerance between the output power of the CDMA handset transmitter and the expected receive power at the base station. This may lead to a gross jump in the output power of the CDMA handset transmitter to catch the next permissible power slot and, thus, may undesirably increase the output power sent to the base station. Such a gross increase in output power may increase interference to other users, which may lead to a reduction in the number of subscribers at a given moment. Therefore, there is a need in the industry to address the problem associated with different industry standards.
After initiating the call, the CDMA handset switches from open-loop mode to closed-loop mode. In closed-loop mode, it is desirable to change the gain of the radio frequency (RF) circuitry at different output levels to accommodate a good signal-to-noise ratio. These changes in the gain of the RF circuitry are often abrupt and cause nonlinearity in the output power. This condition may result in a sudden jump in the output power of the handset rather than a smooth increase. These sudden jumps on the output power may further result in excess output power, that in turn, undesirably increases the output power of the CDMA handset transmitter. Such increases in output power may create more interference and, hence, fewer subscribers are able to connect at any given time. Therefore, there is an unaddressed need in the industry to compensate for these abrupt changes in the gain of the RF circuitry.
The invention provides a variable-slope, variable-gain amplifier (VSVGA) system and method for efficiently and accurately varying amplifier output power. In the preferred embodiment, a VSVGA circuit is employed in a Code Division Multiple Access (CDMA) handset transmitter. Specifically, the VSVGA circuit allows for varying the slope of a line which models the output power transfer function, thus varying the amplifier gain and, hence, the resolution of the output power of a variable-gain amplifier (VGA).
Briefly described, the CDMA handset is configured to receive power change signals from a base station and, in response to those power change signals, to adjust the output power of the CDMA handset transmitter using a control signal that varies the gain of the VGA. By varying the amplifier input control voltage to output power transfer function, which may be approximated by a straight line having a slope which may be varied, the gain of the amplifier can be varied. This adjustment in the gain allows for manipulation of the resolution of the output power.
The CDMA handset comprises the VSVGA circuit in addition to the basic components of a closed-loop power-controlled CDMA handset (e.g. VGA circuitry, one or more mixers, local oscillators, filtering and other processing circuitry, driver circuitry, power amplifier circuitry, conversion circuitry, and an antenna). The VSVGA circuit is configured to manipulate the input control voltage of the VGA, thus, adjusting the gain of the amplifier by varying the slope of the amplifier output.
The invention can also be viewed as providing a method for varying the output power transfer function (gain) of a power amplifier. The method can be broadly conceptualized by the following steps: receiving a system change command (e.g. a command to determine whether the CDMA handset is an IS95A, IS95B, or WCDMA standard) at a mobile unit from a base station and adjusting the output power gain resolution of the amplifier by varying the output power transfer function, as approximated by varying the slope of a straight line corresponding to the output power transfer function. The method may further include the step of linearizing a nonlinear output power of a CDMA handset transmitter in response to abrupt change in the gain of the transmitter""s radio-frequency (RF) circuit.
Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.